Cold seal cohesives are becoming the seal medium of choice in today's packaging industry. Cold seals are particularly useful in wrapping products which are heat sensitive, for example, confectioneries, ice cream and bakery and dairy products. When compared to heat seal materials, cold seal cohesives offer faster packaging speeds and improved seal quality and consistency. Cold seals also have the advantage of not causing the packaging substrate to melt during an equipment shutdown or stoppage. Cold seals also provide energy and time savings associated with the operation, maintenance and control of the sealing equipment, relative to energy and time needed in the heat seal industry.
Cold seals constitute blends of natural rubber latex, e.g., cis-polyisoprene, with a synthetic polymer adhesive such as an ethylene vinyl acetate (EVA) adhesive or an acrylic adhesive. Cold seals are water-based emulsions which may contain minor ingredients such as wetting agents, stabilizers, defoamers, anti-blocks, colorants, etc.
Cold seal latexes are usually applied by either direct gravure, roller coat, spray or Meyer rod applications to a flexible packaging substrate in a pattern about 1/2 inch wide which surrounds the perimeter of the product to be wrapped. The volatile components of a cold seal cohesive, which mainly constitute water, are normally removed by forced hot air such that a solid layer of material between about 2 to about 4.5 pounds per ream is left on the substrate surface. The cold seal can be joined to itself, thus forming a packaging enclosure, by applying pressure for a period of time at ambient temperatures. When applied to itself, this type of closure method is commonly referred to as "cohesive" or "self" sealing.
Cold seal latexes are typically applied to a cold seal receptive surface of a laminated packaging film. In order for a cold seal latex to be useful as a closure, the emulsion must have good wetting when applied to the cold seal receptive surface, as well as strong anchorage (adhesion) when dried. In addition, the cold seal must demonstrate a strong affinity to itself (cohesion) under applied pressure for a length of time.
In the packaging industry there has been an increasing interest in obtaining improved anchorage of cold seal cohesives to receptive laminate film surfaces for the purpose of achieving stronger, particularly air-tight, closures. Hermetic packaging seals insure better package protection and shelf life by helping to keep moisture, air, and odors from entering or exiting the atmosphere surrounding a packaged product. Increasing the anchorage between the cohesive and the substrate is an important aspect for attaining a hermetic seal. Improving the hermetic quality of a package is important to achieving barrier properties necessary in today's market, particularly where low-fat and non-fat food products are becoming more prevalent.
The attainment of a secure cohesive closure is not only dependent on the factors mentioned above but also on such variables as coating weight (cohesive thickness), drying conditions, and sealing equipment and parameters including dwell time, pressure and seal jaw design and alignment. Commercially available non-cavitated biaxially oriented polypropylene (BOPP) cold seal receptive surfaces include treated ethylene-propylene random copolymer surfaces (EPRCP), ethylene-propylene block copolymer treated surfaces, acrylic coated BOPP film surfaces, untreated ethylene-propylene random copolymer surfaces, and polyvinylidine chloride (PVDC) coated surfaces. PVDC and acrylic coated films require additional coating steps which add significant costs to the resultant products. Monolayer slip-modified homopolymer polypropylene films have also been used for cold seal receptivity applications but require a printed or primed surface to ensure secure anchorage.
A need therefore exists for a cold seal receptive surface which can be extruded as a film and requires no surface treatment to attain high levels of cold seal anchorage. A need also exists for a cold seal receptive surface formulation which consists of raw materials completely approved by the United States Food and Drug Administration.